1. Field of the Invention
This invention relates to a positioning control system which ensures accurate positioning of a machine under numerical control.
2. Description of the Prior Art
The positioning of a machine under numerical control is generally achieved by effecting accelerating and decelerating control at the starting and stopping of the machine in consideration of the characteristics of a drive motor, the torque and inertia of the machine, etc. Such accelerating-decelerating control is usually performed linearly or in the manner of an exponential function. In the linear accelerating and decelerating control, for example, as shown in FIG. 1, a feed speed f is linearly increased up to a predetermined value as time t passes after the starting of the machine and held constant at the predetermined value and then linearly decreased to zero after the machine has been moved to the vicinity of its commanded stopping position. In the case where the positioning distance, i.e. the distance between the current position and the commanded stopping position of the machine is short, the feed speed f is reduced as indicated by the broken line.
Also it is known in the art to change the feed accelerating and decelerating speeds with the positioning distance, as shown in FIG. 2. Where the positioning distance is long, the control characteristic is substantially the same as shown in FIG. 1; but where the positioning distance is short, the feed accelerating and decelerating speeds are both increased and the deceleration may in a certain case start before the accelerating speed reaches its predetermined value.
In the positioning operation, however, vibration is sometimes caused, at the end of positioning, by the characteristic of the drive motor and various elements of the machine to make accurate positioning impossible. To prevent such an adverse influence of vibration, it has also been proposed to adopt the combined use of the linear and the exponential-function type decelerating characteristic. With this method, however, the positioning time becomes longer, so that accurate and high-speed positioning is difficult in the case of a machine tool requiring high-speed positioning, such as, for example, a punch press.